The book is probably still fairly up to date. For a variety of reasons, MySQL is changing at a significantly slower pace than PHP. Additionally, a lot of the changes to MySQL are "under the hood", and not actually things that will impact your queries.

The only new features in 5.6 that I've come across so far are greatly improved support for spatial queries.

PHP was stagnant for a while because they were working on PHP 6. Then that PHP 6 was cancelled, and all of the features that had already been built up for it were instead put into 5.3, 5.4 and 5.5. So as a result, all three of those releases came / are coming in quick succession, and all of them have significant new features. This makes PHP resources get outdated very quickly, but that's a lot less true of MySQL.